1. Field of the Invention
The present invention relates to a dielectric ceramic composition, for example, used as a dielectric layer of a multilayer ceramic capacitor; and an electronic device, such as a multilayer ceramic capacitor using the dielectric ceramic composition as a dielectric layer.
2. Description of the Related Art
In the related art, it has been considered to be effective to secure a high permittivity and a preferable temperature characteristic that a dielectric layer of a multilayer ceramic capacitor as an example of electronic devices is formed by dielectric particles (crystal grains) having a core-shell structure, wherein a shell as a region of barium titanate as a main component dispersed with a subcomponent exists on a surface of a core substantially made by barium titanate not dispersed with any subcomponent. A variety of proposals have been made on a technique of giving the core-shell structure to crystal grains.
For example, the patent article 1 (the Japanese Unexamined Patent Publication No. 10-308321) discloses a technique of adding Mg as a subcomponent to barium titanate as a main component and adjusting a firing temperature and firing time, etc. to control a depth of Mg dispersing from crystal grain surfaces to its inside (that is, a shell thickness).
However, in the technique in the patent article 1, particle diameters of the dielectric particles composing a dielectric layer were approximately uniform and the shell thicknesses in the dielectric particles were also uniform, so that the temperature characteristic of the capacitance was preferable but the permittivity was liable to be small.
Thus, to overcome the disadvantage, the patent article 2 (the Japanese Unexamined Patent Publication No. 2004-111951) discloses a technique of making the shell thickness different in accordance with a particle diameter of the dielectric particles. Also, the patent article 2 describes that not all of the dielectric particles have to be covered with the shell and a dielectric particle with its core partially exposed to be open may be included.
The patent article 3 (the Japanese Unexamined Patent Publication No. 2001-313225) also discloses the technique of including dielectric particles with partially open cores. According to the patent article 3, by including the dielectric particles having partially open cores, a volume of the entire shell can be reduced and a permittivity of the entire ceramic sintered body can become high comparing with that in dielectric particles with cores covered entirely with the shell. As a result, a thickness of the dielectric layer can be as thick as possible without reducing the capacitance, so that it gives effects of responding to two opposing demands at a time: a demand for obtaining a thin layer with a large capacity and a demand for attaining reliability and a long lifetime.
When including the dielectric particles with partially exposed cores as disclosed in the patent articles 2 and 3, however, the TC bias characteristic deteriorates in some cases. The reason is not always clear, but it is considered because a voltage is applied to the core as a ferroelectric. When the TC bias characteristic deteriorates, there arises a disadvantage that the capacitance declines in an actual use and the product value is remarkably reduced.